Band pass network



Dec. 24, 1935. N, M. RU T 2,025,128

BAND PAss NETWORK Filed Sept. 26, 1933 4 Sheets-Sheet 1 121 2 v [Zr/l4)7) lNVENTOfi NOEL M. RUST BY/fqg 990, 000 1,000,000 00/0000 ATTORNEYDec. 24, 1935. N RUST I 2,025,128

BAND PAS S NETWORK Filed Sept. 26, 1933 4 Sheets-Sheet 5 fiEAAT/I fRESPONSE K106 u/v/zsj INVENTOR NOEL M. RUST ATTORNEY Dec. 24, 1935. N.M. RUST 2,25,128

' BAND PASS NETWORK Filed Sept. 26, 1953 4 Sheets-Sheet 4 R v [1 u I /lINVENTOR NOEL M. RUST ATTORNEY Patented Dec. 24, 1935 UNITD STATES BANDPASS NETWORK Noel Meyer Rust, Chelmsford, England, assignor to RadioCorporation of America, a corporation of Delaware Application September26, 1933, Serial No. 690,989

In Great Britain October 8, 1932 2 Claims.

This invention relates to frequency selective electrical circuitarrangements and filters. Though not exclusively confined thereto theinvention is particularly advantageous when applied to provide improvedcircuit arrangements of the band pass filter type i. e. adapted to givea response which is approximately constant over a predetermined band offrequencies and which falls away rapidly on either side of the limits ofthat band of frequencies.

Such band pass filters are frequently required for a variety ofdifferent purposes; for example, in radio receivers it is commonlyrequired to provide a turnable band pass filter adapted to giveapproximately constant response over a band of frequencies of widthapproximately equal to that required for speech or music modulation, theresponse falling away rapidly on either side of the limits of this band.Common requirements at the present time are that the width of the bandpassed shall be about 10,000 cycles, and that the position of the bandin the frequency spectrum shall be movable by tuning over the relativelywide range of frequencies occupied bythe so-called lower or higherbroadcast range. These requirements are difficult to satisfy withrelatively cheap and simple apparatus it being more particularlydifficult to secure sharp cut-off effects on either side of the acceptedband, and, as will be readily appreciated such sharp cut-off effects arehighly desirable for the purpose of minimizing interference due to thereception of more than one transmission station at a time. 1

The present invention though applicable to socalled radio frequencycircuits i. e. to circuits performing selective functions at receivedradio frequencies is most advantageously applicable to so-calledintermediate frequency circuits in superheterodyne receivers i. e. tocircuits performing selective functions at the (fixed) intermediatefrequency of a superheterndyne receiver.

The primary object of the invention is to provide frequency selectiveelectrical circuit arrangements giving sharp tuning and rejector effectsor sharp band-pass tuning effects i. e., band pass tuning effects withsharp cut-offs at the ends of the band to be passed.

The invention, though not limited to its application thereto, isparticularly adapted to the provision of a complex resonant circuitarrangement in the intermediate frequency amplifier of a superheterodynereceiver giving a so-ca led band pass filter effect whereby a desiredband of frequencies' will be passed by said amplifier with cut-offeffect occurs.

clearly defined cut-01f effects on either side of the central frequencyof that band.

The invention is'illustrated in and further explained in connection withthe drawings accom panying the present application.

As is well known if there be provided in association in a network, twotuned circuits and a third acceptor circuit of suitable value a socalledband-pass filter effect can be obtained. For example, referring toFigure 2 if a network consisting of two tuned circuits each comprisingan inductance (L1M) and a condenser C1 associated therewith, and with anacceptor circuit consisting of an inductance M and a condenser C, thenif the two tuned circuits (L1M)C1 be similar and the acceptor circuit MCsuitably proportioned a comparatively sharp cut-off effect may beobtained by reason of the said circuit MC, the remaining circuits givinga resonant peak effect at a predetermined distance in the frequencyscale from the frequency at which the The terminals I, 2, represent theinput terminals of the network shown in Figure 2, and the terminals 3,4, the output terminals.

By an important feature of the present invention there is provided animprovement in the fundamental type of circuit exemplified by Figure 2,the improvement resulting in the obtaining of sharper cut-off effectsand better resonant voltage response. In the known fundamental circuitshown in the said Figure 2 an actual inductance M is utilized to providerejector effects, and in essence the present feature of the inventionconsists in so modifying the fundamental circuit shown in the saidFigure 2 that the inductance utilized to provide the rejector effects isa mutual inductance instead of an actual physical inductance. Thus theinductance M of the circuit MC of the said Fi ure 2 is constituted inaccordance with this feature of the invention not by a separate coil.but by the mutual inductance between two coils each of which is in oneof the two remaining tuned circuits. The use of mutual inductance inthis way produces sharper selectivity characteristics than obtainable inthe ordinary way by reason of the fact that, where mutual inductance isso employed, coil losses (energy losses) present in ordinary knownarrangements are reduced.

Figure 1 shows the circuit of Figure 2, modified in accordance with thepresent invention, and as will be seen the modified circuit consists ofcoils in and, condensers C1 the coils being similar and forming a tunedcircuit. The junction of the two coils L1 and the junction of the twocondensers C1 are connected together through the fixed condenser C. Thetwo coils L1 are mutually coupled, the mutual inductance not beingrepresented in Figure 1, and this mutual inductance M replaces, inaccordance with the present invention, the actual coil M of Figure 2.

Figure 3 shows in conventional circuit diagram form, and Figure 4 inwhat may be regarded as an electrically equivalent diagram form, anactual practical arrangement in accordancewith the present invention andsuitable for use, for example, in connection with radio reception.Referring to the said Figure 3, it will be seen that there is an aerialcircuit which is coupled to a complex resonant network comprising twoinductance coils designated L1; two inductance coilsdesignated L2; twovariable condensers designated 01; a third variable condenser designated01, and two fixed condensersdesignated 02.

The arrangement of the said Figure 3 may be regarded as an extension ofthe arrangement of Figure 1 to two stages. As will be seen, thearrangement of the said Figure 3 includes three series connected tunedcircuits 01(L1-M1) 01(L2M1)+(L2M2); and 01'(L1M2). These three circuitsare tuned to the fundamental frequency e. g. to 1,000,000 cycles persecond in the case of a radio receiver; in other words these threecircuits are tuned to the middle frequency of the band desired to bepassed. M1 represents the mutual inductance between the coils L1 andLzas represented to the left of Figure 3, and M2 represents the mutualinductance between the coils L2 and L1, as represented to the right ofthe said Figure 3. The acceptor circuits constituted by the mutualinductance M1 with the condenser 02, and the mutual inductance M2 withthe condenser 02 are tuned to frequencies on ,either side of thefundamental or central frequency e. g. to take the numerical exampleabove given M102 may be tuned to 990,000 cycles per second, and

M202 may be tuned to 1,010,000 cycles per second.

It is thought that from the description already given, the relation ofthe said Figure 3 to Figure 4 will be evident from the drawings. Figure5 shows graphically therelation between the ratio It will be noted thatin the arrangement shown in the said Figure 3 there is employed what maybe termed a.mutual inductance acceptor circuit i. e. an acceptor circuitin which the inductance is a mutual inductance'as distinct from aself-inductance, this aoceptor'circuit being utilized in conjunctionwith other resonant circuits to produce (in the case of the said Figure3) a band pass effect. As above stated, the use of a mutual inductanceinstead of (as hitherto) a self inductance to produce rejector eflectSconstitutes an important feature of this invention, and in theembodiments so far described the mutual inductance acts as an auxiliarybetween two coupled circuits, the arrangements so far described being ofthe band pass type. Figure 6 of the accompanying drawings showsdiagrammatically an arrangement involving the same fundamentalprinciples but wherein the mutual inductance is not employed as acoupling link..

Referring to Figure 6, the network therein shown consists of twoinductance coils l1 and L1 in series With one another between terminalsI and 3, and two condensers 01 and. 02, the former being connectedbetween a conductor joining terminals 2 and land the junction points ofcoils Z1 and L1, and the latter being connected between the terminals 3and 4. The coils Z1 and L1 are coupled together to have a mutualinductance M1 equal to the inductance of the coil Z1. The combinationM101 is resonant to a frequency to be rejected, and the condenser 02resonates with V L1-M at a frequency to be accepted. Figure 'I is thetheoretically equivalent diagram corresponding to Figure 6. v

As will be apparent to those skilled in the art, a circuit as shown inthe accompanying Figure 6 will have the property of cutting out anundesired frequency, namely, that at which the combination M101 isresonant. The said circuit is, therefore, well adapted for use in theradio frequency selecting stage or stages of a superheterodyne receiver.As is well known, a difficulty frequently met with in superheterodynereceivers is that known as second channel interference namely that formof interference which is due to the fact that there are two radiofrequencies which when heterodyned by the local oscillator frequencywill produce the intermediate frequency. An arrangement as shown in theaccompanying Figure 6 may be employed as a tunable device in the radiofrequency selective stage or stages of a superheterodyne receiver toeliminate such second channel interference.

A development of. the circuit shown in Figure 6 is illustrated intheacco-mpanying Figure 8 and it is thought that the arrangement ofFigure 8 will be obvious therefrom having regard to the referencesthereon. The accompanying Figure 9 is the equivalent theoretical circuitcorresponding to Figure 8. In the arrangement of Figure 8 M101 isresonant to one frequency, M202 is rescnant to another and((L1M1)+(L2M2) )02 is resonant to a third which is intermediate thefirst two, the said third frequency being an accepted frequency i. e.afrequency at which maximum response is obtained, and the first twobeing rejected or dipfrequencies i. e. frequencies at which minimumresponse is obtained. The characteristic obtained with the, circuit ofFigure 8 is therefore ahumped characteristic with steeply sloping sidesrising from the two dip frequencies. 7 1

Figure 12 shows another circuit. arrangement in accordance with thisinvention, and suitable for use for cutting out interference at one sideor another of a particular desired frequency-e. g. for eliminatingsecond channel interference in a superheterodyne receiver for whichpurpose the said circuit arrangement would be provided in the radiofrequency portion of the receiver as in the case of the circuit shown inthe accompany ing Figure 6. In the accompanying Figure 12, L1 aresimilar inductances coupled together by a mutual inductance M and 0102are condensers.

The accompanying Figure 13 is the equivalent theoretical circuitcorrespondence to Figure 12. It is possible to employ in cascade (inseparate valve stages) two differently adjusted circuit arrangements,each adapted to cut out interference to one or other side of a givenfrequency, and thus to produce a band pass efiect e. g. two arrangementsas illustrated in the accompanying Figure 12 may be so employed.

Figure 14 shows such an arrangement. In the said Figure 14, V1 and V:are thermionic valves in cascade, and it will be observed that there isassociated. with the plate circuit of the valve V1 a filter generallydesignated I (said filter being as illustrated in the accompanyingFigure 12) while a similar but differently adjusted filter 2 isassociated with the valve V2. Ch are chokes. IN are the input terminalsand U are the output terminals. The circuit I is so adjusted as to havea response curve as shown at IR in the accompanying Figure 15, while theresponse curve of circuit 2 is shown at 2R, the resultant overallresponse curve being shown at OR.

Another simple arrangement in accordance with the invention is shown inthe accompanying Figure 16, the accompanying Figure 1'7 being thetheoretical equivalent circuit. In these two figures M is the mutualinductance between inductances L1 and L2 and also that between theinductances La and L4. The arrangement of the said Figure 16 is ineffect that of two series resonant circuits in parallel, one of thesecircuits producing a dip at a frequency f1 and the other producing a dipat a frequency ii, there being a peak resonance at a frequency Thisbroad principle of using two acceptor circuits to produce a band passeffect is disclosed in British specification No. 19968/ 32, but whereasin the arrangements shown in the said specification the sharpness of theeffect produced is obtained by applying negative resistance by means ofvalves, the use of mutual inductance in the present case enables theprovision of valves for negative resistance to be dispensed with. In theaccompanying Figure 16, the interconnection between the mutual shuntarms is made by a condenser which resonates with the inductance (L2M)and (Ls-M).

Figures 19 to 24 are illustrative of still further arrangements inaccordance with this invention. These further arrangements may beregarded as examples of cases in which the coupling element or elementsin a band pass filter circuit arrangement of the kind employing aplurality of circuit elements is so designed as to present substantiallyvarying reactances for different frequencies in the frequency spectrumconsidered, and the magnitudes of the components in the coupling elementrelative to the other components of the whole circuit arrangement are sochosen that at resonance the coupling is approximately at or above thevalue giving optimum coupling condition, continues approximately at orabove the said value for that range of frequency over which anapproximately flat topped response curve is required and decreasesrelatively rapidly from the said value after the desired flat toppedregion is passed. Figure 18 exemplifies the known tunable band passfilters consisting in essence of two elemental circuits coupled togetherby a coupling impedance, the complex arrangement having a plurality ofmodes of oscillation and a plurality of resonant frequencies which arethe principal factors in determining the response curve of the wholearrangement.

The circuits of Figures 19 to 24 may be regarded as improvements in thetype of filter exemplified by Figure 18, although the separate circuitelements in arrangements, as typified by the said Figure 18, may be ofrelatively complex form, such arrangements in general may betheoretically reduced so as to be represented by the simple theoreticalcircuit shown in the said Figure 18, in which figure the condenser C,resistance R, and inductance L to one side of the impedance X are thecomponent portions of one circuit element, the corresponding capacity C,resistance R, and inductance L to the other side of the'impedance Xbeing the component portions of the other circuit element. X is thecoupling impedance between the circuit elements. It may be shown thatwith a band pass filter of the general type exemplified theoretically bythe said Figure 18, whether the circuit elements be coupled directly orby mutual inductance, condenser coupling or in any other way, if thevalue of the impedance X at any particular frequency be chosen relativeto the resistance R at such value as will produce the requiredsubstantial fiat-topped response curve, that choice will determine thesh-ape of the sides of the response curve and the positions of thecut-off frequencies and in arrangements as hitherto proposed, whereinthe coupling impedance X has been designed to be approximately ofconstant impedance over the frequency spectrum considered the adjustmentof X relative to R determines the side outoff frequencies, and the shapeof the sides of the response curve and unless further circuits areintroduced sharper cut-off effects cannot be obtained.

The expression optimum coupling condition employed above means thatcondition which is obtained when the value of the reactance presented bythe coupling impedance X is equal to the value of the resistance R. Solong as the reactance X is equal to or greater than the resistance R theover-all eficiency will be high. The more rapidly the ratio decreasesthe sharper will be the cut-off effects. Figure 19 shows an arrangementin which the coupling impedance is constituted by a tuned circuit LzCzdesigned to have a resonant period at the same frequency as those of theside circuits incorporating the reactances L101. stants of the circuitLzCz are so chosen that its reactance is comparable to the value of Rand preferably greater than R in order to obtain a close approximationto a flat topped response curve. ly flat top will be obtained over thatregion where the reactance presented by the impedance L2 and C2 inparallel is greater than R and when this region has been passed theratio The con- With this arrangement the approximatelow losses.

4 7 peak is produced at the frequency at which L1 and C1 resonate andvery low impedance dips are produced at frequencies on either side. Theratios & e V

. the coupling element.

Figure 21 shows a further modified arrangement which may be regarded asclosely equivalent to the arrangement of Figure 20, but wherein mutualinductance is utilized, the arrangement of Figure 21 thus presenting theadvantage of very The resemblance between Figure 21 and Figures 8 and llwill be noted.

In Figure 21 M1 represents the coupling between L1 and Z1 and M2 thecoupling between L2 and 22. R1 and R2 are resistances. The simplest caseof an'arrangement as shown in Figure 21 is where M1 equals 11 and M2equals Z2 but good effects can be obtained even if this condition be notcomplied with. As M1 and M2 would not in practice differ very much forcases where high selectivity is required, it will generally be ,possibleto make L1 equal to L2.

The equivalent theoretical circuit for the case where M1 equals 11 and'M2 equals 12 is represented in Figure 22. Figure 23 shows a threecircuit element arrangement wherein one mutual inductance M1 is utilizedto sharpen the general cutoff effect on one side of the desired responsecurve and the other mutual inductance M2 is applied to sharpen thegeneral'cut-ofi effect on the other. In the said Figure 23, as will beseen, there are two coupling impedances, one between each pair ofsuccessive circuit elements and, as

7 before, these coupling impedances are so chosen that the reactancespresented thereby are approximately equal to and preferably greater thanthe resistance value R over the range through which a flat toppedresponse curve is required 'but falls away relatively rapidly on eitherside of that range. I v

The diagram in the accompanying Figure 11 shows a circuit arrangementillustrated as energized from a tetrode valve V. In the diagram ofFigure 11 the condensers C1 and C2 are shown variable, and when theapparatus was adjusted and arranged to give the various values marked onthe said diagram the series of characteristic curves shown graphicallyin Figure 10 were obtained. In the graphs of the said Figure 10 theabscissac are frequencies in KC and the ordinates response values indecibels and also in voltage ratio (output terminals to input terminals)The three different characteristics were obtained for threedifferentsettings of the condensers C102 and it will be observed thatthe adjusting of these condensers results in altering the positions ofthe dip frequencies on the frequency scale substantially withoutaltering either the flatness of the top of the curve, the heightthereof, or the steepness of the sides; in other words adjustment of C1and'Cz results in altering practically only the width of band passed.

It will be observed from the accompanying Figure 11 that the responsecurve rises outwardly of the dip frequencies. This can be avoided byassociating with a circuit as shown in the said Figure 11 an ordinarytuned circuit; e. g. an ordinary tuned circuit may be connected incascade with a filter in accordance with this invention. By suitablydimensioning the tuned circult and, in particular, the damping thereofso that it has a characteristic of about the same (or somewhat higher)maximum height as the filter characteristic, a combined characteristicgenerally resembling that of the filter alone, except that 15 it doesnot rise to any very great extent out- 7 wardly of the dip frequencies,may be obtained;

As will be apparent to those skilled in the art, careful screeningshould be adopted in carrying the invention into practice to preventundesired I coupling occurring. The present invention is of generalapplication, but one particularly advantageous use of band pass filtersin accordance with the invention is in connection with the intermediatefrequency stages of superheterodyne receivers and by providing aband-pass circuit arrangement in accordance with this invention inassociation with such an intermediate frequency amplifier a verysatisfactory band-pass efi'ect may be obtained in the amplifier.

What is claimed is:

1. In combination with a source of side band modulated carrier energy, aband pass selector network comprising at least three series connectedcircuits each tuned to the frequency of said carrier, each circuitincluding a coil and a tuning condenser, mutual inductance between thecoils of each pair of adjacent circuits coupling them, each mutualinductance having in series therewith an auxiliary condenser to providea 40 series resonant path, whereby two such paths are provided, one ofthe paths being tuned to a frequency differing from the carrierfrequency by the width of one side band, and the other path being tunedto a frequency differing from the carrier frequency by the width of theremaining side band.

2. In combination, a source of side band modulated radio frequencycarrier energy, a selector network comprising a pair of tuned circuits,each circuit including a coil and a tuning condenser, said circuitsbeing tuned to the carrier frequency, said coils being connected inseries and mag- V netically coupled, an auxiliary condenser beingconnected in series with the mutual inductance between said coils andproviding therewith a series resonant path, an electron discharge tubehaving its input electrodes coupled to the output of said network, asecond network similar 'in all respects to the first network, coupled tothe 6 output electrodes of the tube, the series resonant paths of saidnetworks each being tuned to a frequency, one above and the other below,and differing from said carrier by the width of the side band frequency.

NOEL MEYER. RUST.

